A bonding pad (or just pad), or a bonding structure, serves to enable electrical contact via a wire contact of an electrical device on a processed wafer, or a chip, for example. A corresponding process is also termed wire bonding, bonding process or simply just bonding. In general, the requirements placed upon the top metal sheet are a high current-carrying capability and a low resistance. In order to meet these requirements, the thickness of the conductor line is crucial, above all, apart from the suitable choice of material. Since the last conductor line plane and the pad are usually realized in the same plane for reasons of cost reduction, the pad metallization also has the thickness necessitated for the conductor lines. If the metallization comprises a soft metal (e.g., Al, AlCu or AlSiCu), difficulties in the bonding on the pad may arise upon exceeding a critical metallization thickness.
Extensive research on wire bonding has shown that passivation cracks develop in the bonding pads, or that a thick top aluminum layer is so soft that it is deformed during wire bonding. In order to avoid this, the force with which a bond wire is welded on the bonding pad would have to be reduced or decreased. However, this has the consequence that no sound connection able to carry a load is achieved between the wire and the bonding pad, and this failure mechanism is called “Non Stick on Pad” (NSOP).
A disadvantage of passivation cracks is a reduced robustness with respect to moisture. In very small housings employed in single wafers such a decrease in quality is unacceptable.
The technical problem thus consists of a lack of a process window for wire bonding on thick layers that can occur when using aluminum and are deformed during the bonding process.
When using gold, this problem does not occur since gold comprises a particularly high current-carrying capability, and thus, correspondingly thinner layer thicknesses may be used for the top metal sheet. However, on 8-inch wafers an Au-lift-off process cannot be fabricated in a stable manner since it is used, among other things, not only for generating the bonding pads but also for hooking up transistor fingers. Thus, not only a coarse structure of the bonding pads has to be generated, but also a fine structuring for terminals of the transistor fingers has to be ensured.